This invention relates generally to phase shifters, and is particularly concerned with a phase shifter which can provide a substantially linear variable phase shifting characteristic with a relatively constant amplitude, i.e. a phase shifter which does not introduce spurious amplitude changes of the phase shifted signal.
It is known to provide a vector modulator for providing a controlled phase shift and amplitude gain (or attenuation) for a supplied signal; for example a vector modulator can comprise a variable phase shifter and a variable gain amplifier. Such a vector modulator can for example be provided in a loop with a tuning element, such as a surface acoustic wave device, to provide an oscillator.
In order to provide a versatile form for such a vector modulator, it is desirable for it to provide separate and independent control of the phase shift and gain which it provides. While it is relatively easy to provide a variable gain amplifier which does not introduce substantial phase changes, it is relatively difficult to provide a phase shifter which provides a variable phase shift without introducing substantial amplitude changes. This difficulty is increased by a requirement for wide band operation of the phase shifter with a substantially linear relationship between a phase shift control voltage and the resulting phase shift, especially over a relatively large range of phase shifts, and may be further increased by other requirements, such as for stability against temperature and/or production process variations.
A need exists, therefore, to provide an improved phase shifter.
According to one aspect, this invention provides a phase shifter comprising: a phase splitter for producing two phase quadrature signals from an input signal; a weighting circuit for producing two weighting signals in accordance with sine and cosine functions of a control signal; two multipliers for multiplying each of the two phase quadrature signals by a respective one of the two weighting signals to produce a respective one of two products; and a summing circuit for summing the two products to produce an output signal which is phase shifted from the input signal in dependence upon the control signal.
Thus sine and cosine weighting functions are applied to phase quadrature versions of the input signal to be phase shifted, and the weighted results are summed to provide a phase shifted output signal with an amplitude which is relatively independent of the phase shift.
The input signal typically comprises a high frequency signal, for which conveniently the phase splitter comprises a polyphase network and each of the multipliers comprises a four-quadrant multiplier such as a Gilbert Cell multiplier.
Preferably the weighting circuit comprises two translinear sine shaping circuits each having differential current outputs providing a respective one of the two weighting signals from input currents supplied thereto, and a circuit for providing the input currents to the two sine shaping circuits in dependence upon the control signal, the input currents of the two sine shaping circuits being offset relative to one another whereby the differential current outputs of the two sine shaping circuits are provided in accordance with a sine function and a cosine function, respectively, of the control signal.
The circuit for providing the input currents to the two sine shaping circuits in dependence upon the control signal can comprise a circuit for providing a reference current; a differential amplifier responsive to the control signal for producing differential control currents dependent upon the reference current and the control signal; and a plurality of current mirror circuits for producing the input currents to the two sine shaping circuits from the reference current and the differential currents.
The invention also provides an analogue method of phase shifting a high frequency input signal in dependence upon a control signal, comprising the steps of: splitting the input signal into two components in phase quadrature; multiplying the two phase quadrature components by respective weights determined in accordance with sine and cosine functions, respectively, of the control signal to produce respective product signals; and summing the product signals to produce a phase shifted output signal having an amplitude substantially independent of its phase shift.
The step of multiplying the two phase quadrature components by respective weights can comprise producing each of said weights as differential currents and reducing the differential currents by a fixed current; this has the advantageous effect of reducing minimum insertion loss of the phase shifter.
The step of multiplying the two phase quadrature components by respective weights can comprise producing said weights as differential currents in two translinear sine shaping circuits in dependence upon currents supplied to the sine shaping circuits, and producing the currents supplied to the two sine shaping circuits in dependence upon the control signal and offset from one another for the two sine shaping circuits to provide said sine and cosine functions, respectively.
Another aspect of the invention provides a weighting circuit for producing weights in accordance with sine and cosine functions of a control signal, the weighting circuit comprising two similar translinear sine shaping circuits each responsive to supplied currents of (1xe2x88x92X)I, 2I, and (1+X)I, where I is a reference current and X is a control variable within a range xe2x88x921 less than X less than 1, to provide differential currents in accordance with a sine function of X, and a current supply circuit for supplying said currents to the two translinear sine shaping circuits with the variable X for the currents supplied to one of the two translinear sine shaping circuits offset by 0.5 from the variable X for the currents supplied to the other of the two translinear sine shaping circuits to provide said differential currents in accordance with a cosine function instead of a sine function, the differential currents provided by the two translinear sine shaping circuits constituting said weights.
Preferably the current supply circuit comprises a source for the reference current I, a differential amplifier responsive to the control signal for producing differential control currents dependent upon the reference current and the control signal, and a plurality of current mirror circuits for producing the currents supplied to the two translinear sine shaping circuits from the reference current and the differential currents.
The current supply circuit is conveniently arranged for supplying said currents to said one of the two translinear sine shaping circuits with the variable X within a range xe2x88x920.5 less than X less than 0.5 and to said other of the two translinear sine shaping circuits with the variable X within a range 0 less than X less than 1.
The invention further provides a method of producing weights in accordance with sine and cosine functions of a control signal, comprising: producing a first set of currents (3I/2)xe2x88x92Ic, 2I, and (I/2)+Ic, where I is a reference current and Ic is a controlled current less than I and dependent upon the control signal; producing a second set of currents Ixe2x88x92Ic, 2I, and I+Ic; and supplying the first and second sets of currents respectively to first and second similar translinear sine shaping circuits, each of the translinear sine shaping circuits being responsive to supplied currents of (1xe2x88x92X)I, 2I, and (1+X)I respectively to provide differential output currents in accordance with a sine function of a control variable X within a range xe2x88x921 less than X  less than 1, whereby said differential output currents of the first and second translinear sine shaping circuits constitute said weights in accordance with sine and cosine functions of the control signal.